An ADU cost calculator helps homeowners estimate the total expense of building an accessory dwelling unit — also known as a granny flat, in-law suite, or backyard cottage. By selecting the ADU type, square footage, region, and finish level, you can get a realistic cost range that includes construction, permits, utility hookups, and site preparation. The calculator also projects potential rental income and how long it takes to recoup your investment.
ADU Project Details
$100–$200/sqft typical range
Additional Costs
Rental Income (for ROI)
Estimated monthly rent if you plan to rent the ADU
How to Use the ADU Cost Calculator
Building an accessory dwelling unit is one of the most popular ways to add living space and rental income to your property. Whether you are considering a backyard cottage, garage conversion, or basement apartment, this free ADU cost calculator gives you realistic cost estimates and ROI projections so you can plan your budget before talking to contractors.
Step 1: Select Your ADU Type
Choose from five common ADU types: detached new construction (most expensive but most flexible), attached addition, garage conversion, basement conversion (typically cheapest), or prefab/modular ADU. Each type has a different cost-per-square-foot range based on the structural work required. Garage and basement conversions save money because the outer shell already exists, while detached builds require full foundation, framing, roofing, and exterior work.
Step 2: Choose Size and Region
Pick from preset sizes ranging from a 400 sqft studio to a 1,200 sqft two-bedroom unit, or enter a custom size. Then select your cost region. High-cost states like California, New York, Massachusetts, and Washington carry a 1.3-1.5x multiplier due to higher labor rates, material costs, and permitting requirements. Low-cost states like Texas, Florida, Georgia, and Ohio come in at 0.7-0.9x of the national average.
Step 3: Set Finish Level and Additional Costs
Choose between basic, mid-range, and high-end finishes. Basic uses builder-grade materials, while high-end includes premium countertops, fixtures, and appliances. Toggle the additional cost categories — permits and fees, utility connections, site work, and design — to include or exclude them from your estimate. Most ADU projects require all four, but if you already have utility stubs or a prepared site, uncheck those for a more accurate number.
Step 4: Enter Expected Rental Income
If you plan to rent the ADU, enter the expected monthly rent based on comparable units in your area. The calculator uses this to project your payback period — how many years of rental income it takes to recoup the construction cost. A shorter payback period means a better return on your investment.
Step 5: Review Your Results
After clicking Calculate, review the low and high cost estimates, the cost-per-square-foot range, and the projected payback period. The detailed breakdown shows exactly how costs are distributed across construction, permits, utilities, site work, and design. The property value section estimates how much your home value could increase — ADUs typically add 20-30% of their construction cost to overall property value, making them a solid long-term investment.
Frequently Asked Questions
Is this ADU cost calculator free?
Yes, this ADU cost calculator is completely free with no signup or account required. All calculations run locally in your browser. Your project details and financial data are never sent to any server.
Is my data private?
Absolutely. All calculations happen entirely in your web browser using client-side JavaScript. No data is transmitted, stored, or logged anywhere. You can verify this by disconnecting from the internet after the page loads.
How much does it cost to build an ADU?
ADU costs vary widely based on type and location. A garage conversion typically costs $100-200 per square foot, while a detached new construction ADU runs $200-400 per square foot. Add permits ($5,000-15,000), utility connections ($5,000-20,000), and site work ($10,000-30,000) for the total project cost.
What is the cheapest type of ADU to build?
Basement conversions are generally the cheapest at $75-175 per square foot because the structural shell already exists. Garage conversions are the next most affordable at $100-200 per square foot. Both save significantly on foundation and framing costs compared to new construction.
Do I need a permit to build an ADU?
Yes, virtually all jurisdictions require building permits for ADU construction. Permit costs typically range from $5,000 to $15,000 depending on your location. California, Oregon, and Washington have streamlined ADU permitting processes, while other states may have more complex requirements.
How long does it take to recoup ADU construction costs?
Most ADU owners recoup their investment within 5-15 years through rental income, depending on local rental rates and construction costs. In high-rent markets like California, payback periods can be as short as 5-8 years. ADUs also typically increase property value by 20-30% of the construction cost.
What size ADU should I build?
The most popular ADU sizes are 400-800 square feet. A 400-500 sqft studio works well for singles or couples, while 600-800 sqft accommodates a one-bedroom layout. Larger ADUs of 1,000-1,200 sqft can include two bedrooms. Check your local zoning for maximum size limits.
Does an ADU increase property value?
Yes, an ADU typically increases property value by 20-30% of the construction cost immediately, and often more in markets with strong rental demand. Some studies show ADUs can add 20-30% to overall property value. The income-generating potential also makes properties with ADUs more attractive to buyers.